pg_upgrade: "pg_ctl failed to start the new server"

I'm trying to upgrade Postgres 9.0 to 9.1 with pg_upgrade. Both
versions are installed from the PGDG Yum repo:

-bash-4.1$ /usr/pgsql-9.0/bin/postgres -V
postgres (PostgreSQL) 9.0.8
-bash-4.1$ /usr/pgsql-9.1/bin/postgres -V
postgres (PostgreSQL) 9.1.4

I can successfully start and connect to both 9.0 and 9.1. When I run
pg_upgrade, it fails with the error "pg_ctl failed to start the new
server," however it apparently does start the new server (output
below). Any ideas? This is a freshly installed CentOS 6.2 machine
with the 9.0 DB copied over from another machine via streaming
replication (which I have since ended and the server is now
standalone).

Thanks.

# su - postgres
-bash-4.1$ ps ax | grep postg
4354 pts/0 S 0:00 su - postgres
4389 pts/0 S+ 0:00 grep postg
-bash-4.1$ /usr/pgsql-9.1/bin/pg_upgrade -c -b /usr/pgsql-9.0/bin/ -B
/usr/pgsql-9.1/bin/ -d /var/lib/pgsql/9.0/data/ -D
/var/lib/pgsql/9.1/data/
Performing Consistency Checks
-----------------------------
Checking current, bin, and data directories ok
Checking cluster versions ok
Checking database user is a superuser ok
Checking for prepared transactions ok
Checking for reg* system oid user data types ok
Checking for contrib/isn with bigint-passing mismatch ok

pg_ctl failed to start the new server
Failure, exiting
-bash-4.1$ ps ax | grep postg
4354 pts/0 S 0:00 su - postgres
4445 pts/0 S 0:00 /usr/pgsql-9.1/bin/postgres -D
/var/lib/pgsql/9.1/data -p 5432 -b
4446 ? Ss 0:00 postgres: logger process
4448 ? Ss 0:00 postgres: writer process
4449 ? Ss 0:00 postgres: wal writer process
4450 ? Ss 0:00 postgres: stats collector process
4453 pts/0 S+ 0:00 grep postg